The invention relates to circuitry and techniques for providing a stable, high gain amplifier by feed-forward compensation of one or more gain stages thereof without use of compensation capacitors, to thereby reduce power consumption and physical size of the amplifier circuit; the invention also relates to an alternative technique to that shown in U.S. Pat. No. 5,155,447 for combining the use of feed-forward stages and compensation capacitors to stabilize an amplifier.
The state of the art is believed to be generally indicated by U.S. Pat. No. 5,155,447 (Huijsing et al), U.S. Pat. No. 5,485,121 (Huijsing et al), and U.S. Pat. No. 4,559,502 (Huijsing). The basic teaching of the Huijsing et al '447 patent entails first creating a stable two-stage amplifier by capacitively nesting a pair of amplifier stages in cascade, and then creating a stable three-stage amplifier by capacitively nesting the two-stage amplifier and an additional first amplifier stage with a feed-forward path being provided from the additional first stage around the second stage to the third stage to provide the high-frequency response of a two-stage amplifier and the low-frequency gain of a three-stage amplifier. FIG. 7 of the Huijsing et al '447 patent is reproduced herein as prior art FIG. 1. That circuit and other circuits known in the prior art feed signals forward to increase low frequency gain in multi-stage high gain amplifiers, but they also include compensation capacitors to accomplish frequency compensation which stabilizes the high gain amplifier by reducing the frequency response rolloff slope to -20 dB per decade at the unity gain point. For example, in prior art FIG. 1 hereof, feedback compensation capacitors C1 and C2 are utilized to stabilize the three-stage amplifier. (Those skilled in the art know that it often is necessary to provide multiple gain stages to obtain the desired accuracy in many amplifier applications because the high frequency gain obtainable with a single-stage or two-stage amplifier is far too low for many applications. Those skilled in the art also know that the use of multiple gain stages substantially complicates the problems of achieving frequency stability.)
It is desirable to avoid use of compensation capacitors to accomplish frequency compensation of amplifier circuitry because compensation capacitors add delays that result in reduction of gain of the amplifier circuitry. This necessitates increasing the bias currents needed for the gain stages of the amplifier circuitry in order to maintain its desired gain and bandwidth and hence its accuracy. Such increased bias currents result in undesirably increased power dissipation. Furthermore, integrated circuit compensation capacitors require a large amount of semiconductor chip area and therefore increase the physical size and hence the cost of the chip.
Accordingly, it would be highly desirable to provide a stable amplifier circuit of three or more stages, which has very high gain, is stable, and avoids or at least minimizes use of compensation capacitors to accomplish frequency stabilization.